Various implementations, and combinations thereof, are related to optical fiber, and more particularly to optical fiber with multi core sections.
Generally speaking, an optical fiber is a fiber of glass or plastic capable of carrying light along its length and typically comprising a core section surrounded in a cladding, as illustrated in FIG. 1. An optical beam is propagated through the length of fiber 100 via a core 102, confined therein by a cladding 104, which has a lower refractive index than the core.
The core itself can either be single mode or multimode. Multimode fibers support multiple transverse modes, whereas single mode fibers support only one. Typically, multimode fibers are used for communication links over short distances or where high power transmissions are desired. In contrast, single mode fibers are used for long-distance communication links. The relationship between the mode of fiber, the core, and the optical beam is given by the following equation:V=2π×NA×a/λwhere NA is the numerical aperture, a is the radius of the core, and λ is the wavelength of the optical beam. When V is less than or equal to 2.405, the fiber is a single mode fiber. Otherwise, it is a multimode fiber.
When doped with rare-earth ions, such as neodymium or ytterbium, optical fibers can be used as the gain medium in fiber lasers or fiber amplifiers. Such fiber is generally referred to as a gain fiber and the rare-earth ions are doped in the core and/or the cladding. Different laser wavelengths are generated from fibers with different doping ions. For example, approximately 1 and 1.3 microns are achieved with neodymium doped fibers, 1.55 and 2.7 microns from erbium doped fibers, 1 micron from ytterbium doped fibers, and 2 microns from thulium and/or holmium doped fibers.
Different types of gain fibers are designed for use in different fiber lasers, the characteristics of the gain fiber effecting the resulting fiber laser. For example, the use of a double cladding gain fiber increases the output power of a fiber laser. Several existing patents focus on this relationship by attempting to affect the quality and nature of fiber lasers through the development of the gain fiber. By way of example, U.S. Pat. No. 4,829,529, issued to Kafka provides a single mode fiber laser pumped by a coherent high power laser diode source. Kafka attempts to address the issue that the small diameter of single mode fibers limits the ability to couple such fibers to high powered coherent laser diode sources, resulting in low powered lasers, whereas multimode fibers are not so limited, but the resulting lasers have poor beam quality output. Specifically, Kafka discloses laser diode pumped fiber lasers with double cladding. FIG. 2 provides an exemplary embodiment of a fiber 200 as used in Kafka having a double cladding where a core 206 embedded within an inner cladding 204 and an outer cladding 202. The optical fiber used has rare-earth ions doped into the core to provide an active gain medium. A multimode pump laser is coupled to the inner cladding to increase the pump power and excite the rare-earth ions in the core of the fiber. The larger cross section of the inner cladding, in comparison to the core, allows the multimode laser to be coupled to a single mode fiber. As a result, the high pump power of the inner cladding compared to the core pump produces a fiber laser having high output.
By way of another example, U.S. Pat. No. 5,566,196, issued to Scifres, attempts to provide an optical fiber laser or amplifier medium using multimode fibers and having an increased output power without producing nonlinear optical effects such as Brillouin scattering. The fiber lasers and amplifiers of Scifres employ optical fibers with two or more generally parallel, nonconcentric doped core regions, each of which is capable of gain or lasing when optically pumped. An exemplary optical gain fiber according to Scifres is illustrated in FIG. 3. The fiber 300 may be single clad or double clad, the single clad fiber having only the inner cladding 304 where as the double clad fiber additionally has the outer cladding 302. Multiple cores 306 may be embedded in a common cladding region, such as inner cladding 304, or in separate cladding regions. The use of multiple cores spreads the light over a larger area of the fiber, compared with a single mode fiber, and thereby reducing or eliminating the non linear optical effects that would otherwise occur at high light intensities.
The cores of the gain fibers of Kafka and Scifries are formed with a relatively uniform area. The laser is generated and/or amplified by one or more rare-earth ions doped into the core of the fiber. Other ions may be used to transfer energy to the lasing ions. For example, ytterbium ions are sometimes doped into an erbium doped fiber. The resulting lasing ions are erbium and receive energy transferred from the ytterbium after absorbing the pump. However, by forming gain fibers with a relatively uniform area, only one wavelength is generated from each fiber. In other words, the laser only occurs in one transition from the upper level to the lower level from one lasing ion.
Clearly, the development of fiber lasers and fiber amplifiers, such as those described in either Kafka or Scifres, would benefit from optical fibers having non uniform cores, each core section being doped with different rare-earth ions and resulting in a gain fiber capable of generating more than one wavelength.